Method and apparatus for a variable intensity pulsed L.E.D. light

ABSTRACT

Improved method and apparatus for hand-held portable LED illumination. The illumination source includes a plurality of LEDs, and an electrical circuit that selectively applies power from the DC voltage source to the LED units, wherein the illumination source is suitable for handheld portable operation. In some embodiments, the electrical circuit further includes a control circuit for changing a proportion of light output having the first characteristic color spectrum output to that having the second characteristic color spectrum output, and that drives the LEDs with electrical pulses at a frequency high enough that light produced has an appearance to a human user of being continuous rather than pulsed. Still another aspect provides an illumination source including a housing including one or more LEDs; and a control circuit that selectively applies power from a source of electric power to the LEDs, thus controlling a light output color spectrum of the LEDs.

RELATED APPLICATIONS

This application is a Divisional under 37 C.F.R. 1.53(b) of U.S.application Ser. No. 10/299,609 filed Nov. 18, 2002 (U.S. Pat. No.6,808,287); which is a continuation of U.S. application Ser. No.09/978,760 filed Oct. 16, 2001 (U.S. Pat. No. 6,488,390); which is acontinuation of U.S. application Ser. No. 09/627,268 filed Jul. 28, 2000(U.S. Pat. No. 6,305,818); which is a divisional of U.S. applicationSer. No. 09/044,559 filed Mar. 19, 1998 (U.S. Pat. No. 6,095,661); whichapplications are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to the field of lighting, and more specificallyto a method and apparatus of controlling and powering a solid-statelight source such as a light-emitting diode or LED, for a portablebattery-powered flashlight.

BACKGROUND OF THE INVENTION

There is a widespread need for hand-held flashlights and lanterns. Onecommon flashlight includes a two-cell battery for power, an incandescentlamp to emit light, and a simple single-pole switch to connect anddisconnect the battery to the lamp. Other flashlights use other numbersof battery cells in order to provide a voltage suitable for variousparticular conditions. Lanterns often use a fluorescent tube to emitlight. Certain keychain fobs use a pair of hearing-aid cells and ared-light light-emitting diode (LED) in order to provide short-rangelighting such as might be needed to find a keyhole in the dark.

Battery technology is such that as electrical power is withdrawn from abattery cell, the voltage available across a given current load willdecrease. This decreased available voltage across the given load causesreduced light output, gradually dimming the light as the battery chargedepletes.

Further, LEDs have voltage, current, and power parameters that must becontrolled in order to maximize device life. Commonly, acurrent-limiting resistor is placed in series with an LED in order thatonly a portion of the voltage drop from the battery is across the LEDand the rest of the voltage drop is across the resistor. This voltagedrop and corresponding power loss in the resistor is dissipated as wasteheat, which is inefficient for a flashlight which should be designed toemit light.

In addition, it is awkward or difficult to determine the amount ofremaining charge in a battery cell, generally requiring removal of thebattery from the flashlight in order to measure the remaining charge. Inaddition, the cost of a separate measurement device can be a negativefor this market. Some battery cells today include a built-inliquid-crystal indicator for the charge in the cell, but such a solutionrequires a separate measurement device/indicator for each battery, andrequires removal of the battery from the flashlight in order to performthe measurement and observe the indication of remaining power.

SUMMARY OF THE INVENTION

Some embodiments provide a method of providing changeable illuminationof such compact size and low weight as to be suitable for single-handedportable operation by a user. The method includes providing one or moreLEDs having a first characteristic color spectrum output and one or moreLEDs having a second characteristic color spectrum output, wherein thefirst characteristic color spectrum output is different from the secondcharacteristic color spectrum output, selectively applying pulsed powerfrom a DC voltage source to the LEDs, wherein the pulses are ofhigh-enough frequency such that the human eye does not perceive thepulses, and changing a pulse characteristic of the pulsed power in orderto change a proportion of light output having the first characteristiccolor spectrum output to that having the second characteristic colorspectrum output.

Some embodiments provide a portable pulsed LED illumination source ofsuch compact size and low weight as to be suitable for single-handedportable operation by a user. The source includes one or more LEDshaving a first characteristic color spectrum output, one or more LEDshaving a second characteristic color spectrum output, wherein the firstcharacteristic color spectrum output different from the secondcharacteristic color spectrum output, and a control circuit thatcontrols a pulse characteristic to the one or more LEDs having a firstcharacteristic color spectrum output in order to change a proportion oflight output having the first characteristic color spectrum output tothat having the second characteristic color spectrum output.

The present invention provides a method and apparatus for an L.E.D.flashlight or other LED illumination source. In one embodiment, aflashlight is described. The flashlight includes a flashlight housingsuitable for receiving therein and/or mounting thereon at least one DCvoltage source such as a battery. The flashlight also includes alight-emitting diode (LED) housing connected to the flashlight housing,the LED housing including a first plurality of LED units that each emitlight and have a reflector for collimating the emitted light forwardlytherefrom generally along an LED optical axis, the first plurality ofLED units including at least seven individual LED units. The flashlightalso includes a first electrical circuit that selectively applies powerfrom the DC voltage source to the LED units, wherein the flashlight isof such compact size and low weight as to be suitable for single-handedportable operation by a user, the flashlight further having a purpose ofproviding general-purpose illumination.

In one embodiment, the LED optical axis of the first plurality of LEDunits in the flashlight are substantially parallel to one another. Inone such embodiment, the flashlight further includes a second pluralityof LED units that each emit light and have a reflector for collimatingthe emitted light forwardly therefrom generally along an LED opticalaxis, wherein the LED optical axis of the second plurality of LED unitsconverge or diverge from one another forwardly from the housing.

In another embodiment, an optical spread angle of the first plurality ofLED units in the flashlight are substantially equal to one another. Inone such embodiment, the flashlight further includes a second pluralityof LED units that each emit light and have a reflector for collimatingthe emitted light forwardly therefrom generally along an LED opticalaxis, wherein an optical spread angle of the second plurality of LEDunits are substantially equal to one another, and different than theoptical spread angle of the first plurality of LED units.

In yet another embodiment, the LED units are connected in aparallel-series configuration with at least two LED units coupled inparallel to one another and in series with at least one other LED unit,and the DC voltage source includes at least three battery cellsconnected in series.

In still another embodiment, the first electrical circuit furtherincludes a control circuit for maintaining a predetermined light outputlevel of the LED units as a charge on the battery cell varies. In onesuch embodiment, the control circuit maintains an average predeterminedlight output level of the LED units as the charge on the battery cellvaries by increasing a pulse width or a pulse frequency as the charge onthe battery cell decreases. In another such embodiment, the controlcircuit maintains an average predetermined light output level of the LEDunits by measuring a battery voltage and adjusting a pulse width or apulse frequency or both to maintain the average light output at thepredetermined level. In still another such embodiment, the controlcircuit maintains an average predetermined light output level of the LEDunits by measuring an average light output and adjusting a pulse widthor a pulse frequency or both to maintain the measured average lightoutput at the predetermined level.

Another aspect of the present invention provides a flashlight including:(a) a flashlight housing, the housing being suitable for at least one ofreceiving therein and mounting thereon at least one DC voltage sourcethat includes at least one battery cell; (b) a light-emitting diode(LED) housing connected to the flashlight housing, the LED housingincluding one or more first LED units that each emit light and have areflector for collimating the emitted light forwardly therefromgenerally along an LED optical axis; and (c) a first electrical circuitthat selectively applies power from the DC voltage source to the LEDunits, the first electrical circuit further including a control circuitfor maintaining a predetermined light output level of the LED units as acharge on the battery cell varies; wherein the flashlight is of suchcompact size and low weight as to be suitable for single-handed portableoperation by a user, the flashlight further having a purpose ofproviding general-purpose illumination.

In one such embodiment, the first LED units being a first plurality ofLED units, wherein the LED optical axis of the first plurality of LEDunits are substantially parallel to one another. In another suchembodiment, the flashlight further includes a second plurality of LEDunits that each emit light and have a reflector for collimating theemitted light forwardly therefrom generally along an LED optical axis,wherein the LED optical axis of the second plurality of LED unitsconverge or diverge from one another forwardly from the housing.

In another such embodiment, the first LED units are a first plurality ofLED units, wherein an optical spread angle of the first plurality of LEDunits are substantially equal to one another. In yet another suchembodiment, the flashlight further includes a second plurality of LEDunits that each emit light and have a reflector for collimating theemitted light forwardly therefrom generally along an LED optical axis,wherein an optical spread angle of the second plurality of LED units aresubstantially equal to one another, and different than the opticalspread angle of the first plurality of LED units.

Another aspect of the present invention provides a method of providinggeneral-purpose illumination of such compact size and low weight as tobe suitable for single-handed portable operation by a user, includingthe steps of: (a) providing one or more first LED units that each emitlight and have a reflector for collimating the emitted light forwardlytherefrom generally along an LED optical axis; (b) selectively applyingpower from a DC voltage source to the LED units; and (c) maintaining apredetermined light output level of the LED units as a charge on thebattery cell varies by controlling the step (b).

In one embodiment, the step of maintaining maintains an averagepredetermined light output level of the LED units as the charge on thebattery cell varies by increasing a pulse energy or a pulse frequency asthe charge on the battery cell decreases. In another embodiment, thestep of maintaining maintains an average predetermined light outputlevel of the LED units by measuring a battery voltage and adjusting apulse width or a pulse frequency or both to maintain the average lightoutput at the predetermined level. In still another embodiment, the stepof maintaining maintains an average predetermined light output level ofthe LED units by measuring a light output and adjusting a pulse energyor a pulse frequency or both to maintain an average light output at thepredetermined level.

Yet another aspect of the present invention provides an illuminationsource, that includes (a) a light-emitting diode (LED) housing includingone or more LEDs; and (b) a control circuit that selectively appliespower from a source of electric power to the LEDs, the control circuitsubstantially maintaining a light output characteristic of the LEDs as avoltage of the voltage source varies over a range that would otherwisevary the light output characteristic. In one such embodiment, the lightoutput characteristic that is maintained is light output intensity. Inanother such embodiment, the control circuit maintains the light outputintensity of the LED units as the voltage of the DC voltage sourcevaries by increasing a pulse width, a pulse energy, or a pulse frequencyas the voltage of the DC voltage source decreases. In another suchembodiment, the control circuit maintains an average predetermined lightoutput level of the LED units by measuring a voltage and adjusting apulse energy or a pulse frequency or both to maintain the average lightoutput at the predetermined level. In yet another such embodiment, thecontrol circuit maintains an average predetermined light output level ofthe LED units by measuring an average light output and adjusting a pulsewidth or a pulse frequency or both to maintain the measured averagelight output at the predetermined level.

Another aspect of the present invention provides a battery-poweredportable flashlight (100) including: a casing (110) suitable to hold abattery; one or more light-emitting devices (LEDs) (150) mounted to thecasing; a switch (140) mounted to the casing; and a control circuit(130) coupled to the battery, the LEDs, and the switch, wherein thecontrol circuit drives the LEDs with electrical pulses at a frequencyhigh enough that light produced by the LEDs has an appearance to a humanuser of being continuous rather than pulsed, and wherein the LEDs have aproportion of on-time that increases as remaining battery powerdecreases. One such embodiment further includes a feedback circuit thatcontrols the pulses so that light intensity produced by the LEDs, asperceived by the human user, is substantially constant across a greaterrange of battery power or voltage than a corresponding range for whichlight intensity is equally constant without the feedback circuit. In onesuch embodiment, the feedback circuit measures a light output of theLEDs. Another such embodiment further includes abattery-voltage-measuring circuit coupled to the control circuit.

Yet another aspect of the present invention provides a method fordriving battery-powered portable flashlight (100) having a casing (110),a DC power source mounted to the casing, one or more solid-statelight-emitting device (LEDs) (150) mounted to the casing, the methodincluding the steps of: receiving input from a user; and based on thereceived input, generating a series of pulses to drive the LEDs suchthat the LEDs have a proportion of on-time that increases as remainingbattery power decreases.

Still another aspect of the present invention provides an illuminationsource including (a) a light-emitting diode (LED) housing including oneor more LEDs; and (b) a control circuit that selectively applies powerfrom a source of electric power to the one or more LEDs, the controlcircuit maintaining a predetermined light output color spectrum of theone or more LEDs as a voltage of the source of electric power varies. Inone such embodiment, the one or more LEDs comprise one or more LEDshaving a first characteristic color spectrum output and one or more LEDshaving a second characteristic color spectrum output, the firstcharacteristic color spectrum output different from the secondcharacteristic color spectrum output, and the control circuit controls apulse characteristic in order to control the proportion of light outputhaving the first characteristic color spectrum output to that having thesecond characteristic color spectrum output. In another such embodiment,the one or more LEDs comprise one or more LEDs having a characteristiccolor spectrum output that varies based on applied current, and thecontrol circuit controls a pulse current in order to control thecharacteristic color spectrum output.

Yet another aspect of the present invention provides an illuminationsource that includes (a) a light-emitting diode (LED) housing includingone or more LEDs; and (b) a control circuit that selectively appliespower from a source of electric power to the LEDs to adjust a lightoutput color spectrum of the one or more LEDs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of the present invention, a schematicrepresentation of a handheld LED flashlight 100.

FIG. 2 is a circuit block diagram of an LED flashlight circuit 200,which circuit is used in some embodiments of LED flashlight 100 of FIG.1 or LED light source in camcorder 500 of FIG. 5 or other devices suchas machine-vision systems.

FIG. 3 is a circuit block diagram of an LED flashlight circuit 300,which circuit is used in some embodiments of LED flashlight 100 of FIG.1 or LED light source in camcorder 500 of FIG. 5 or other devices suchas machine-vision systems.

FIG. 4 is a circuit block diagram of an LED flashlight circuit 400,which circuit is used in some embodiments of LED flashlight 100 of FIG.1 or LED light source in camcorder 500 of FIG. 5 or other devices suchas machine-vision systems.

FIG. 5 is a diagram showing a controlled LED light source as integratedinto a handheld camcorder 500.

FIG. 6 is a graph of color spectrum versus current for an LED to be usedin one embodiment of the present invention.

FIG. 7 is a circuit block diagram of an LED illumination device circuit700, which circuit is used in some embodiments of LED flashlight 100 ofFIG. 1 or LED light source in camcorder 500 of FIG. 5 or other devicessuch as machine-vision systems.

FIG. 8 is a circuit block diagram of an LED illumination device circuit700 that uses a current mirror.

FIG. 9 is a graph of color spectrum (photoluminescence) versustemperature for an LED to be used in one embodiment of the presentinvention.

FIG. 10 is a circuit block diagram of a machine vision system using anLED illumination device according to the present invention.

FIG. 11 is a circuit block diagram of an LED illumination deviceaccording to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

The present invention takes advantage of the efficiency ofhigh-intensity, light-emitting diodes (LEDs) in the visible spectrumand/or infra-red (IR) or ultra-violet (UV), arranged in variouspatterns, the low-voltage properties of CMOS integrated circuits andcomponents, and the efficiency derived from switching the current to andlimiting the duration of current to the LEDs to project lightefficiently and with constant brightness even as the battery supplyvoltage decays over time. The invention takes advantage of the dynamicimpedance of the LEDs which causes the voltage across the LED to riserapidly relative to the current flow through the LED to limit theinitial current flow to the LED, when battery voltage is highest, toprevent wire bond heating from causing premature failure of the LEDs.The present invention controls the current flow duration (pulse width)to limit power dissipation in the LEDs during the LEDs' on state, andincreasing the pulse width as the battery voltage decreases over time tomaintain substantially constant perceived or average LED intensity overthe course of the battery's life. The invention controls the switchingfrequency of the pulse width to further control the LED intensity andpower dissipation while maintaining a constant light output from theLEDs as perceived or visible to the human eye, or a light-sensingdevice, e.g., camera, night-vision scope, CMOS and CCD sensor and pixelarrays. The present invention provides a compact, portable light source,preferably sized to be readily hand-held, for illuminating an object,several objects, or areas for human use and/or machine operation. In oneembodiment, the invention measures battery voltage and in turn regulatesthe LED intensity. In another embodiment, the present invention uses alight-sensing device such as a light-sensing transistor orlight-detecting diode (LDD) in proximity to the output LED(s) to measurethe average brightness and further regulate the LEDs' output.

Another embodiment of the present invention provides operator-selectablecontrol of the pulse frequency and/or the pulse width to provide areduced apparent brightness in order to increase battery life insituations when maximum brightness is not required. In one suchpulse-frequency embodiment, the apparent (visible) pulse frequency wouldprovide a stroboscope effect for safety or entertainment. In thisembodiment, the visibly interrupted or pulsed pulse train may includerepetitive pulses or a coded sequence as in Morse code “SOS” or apredetermined password or security string of pulses that may then beused as a key or identifier. A further refinement of this embodimentwould provide the user with a method for strobing out a message. It isunderstood that what appears to be a single visible pulse may actuallyinclude a high-frequency series of pulses in order to increase theapparent brightness of a single pulse while also protecting the LEDsfrom excessive power dissipation. In yet another pulse-frequencyembodiment of the invention, a variable or adjustable constant sequencepulse train is established for the accurate measurement of the velocityor frequency of an object in motion or vibration.

Another embodiment of the present invention uses various colored LEDsfor specialized purposes. In one such embodiment, long-wavelength LEDs,660 nm or longer, are used to provide underwater divers or aquariumenthusiasts a light source for observing undersea life at night withoutadversely affecting the nocturnal activities of such wildlife. Thisfunctionality is also useful for tropical aquarium owners who also wishto observe the nocturnal activities of the occupants of their aquariums.In another such embodiment, short-wavelength blue LEDs are used with aUV filter to view fluorescing materials, including but not limited to:taggants, stamps, security codes and security seals. As UV LEDs becomereadily available (such as those announced as made by IBM Corporation inthe Mar. 9, 1998 issue of Electronic Engineering Times, page 39), thesecould be used in place of the blue LEDs. In other embodiments, asuitable LED normally emitting in the blue spectrum, for example madefrom GaN (gallium nitride) or InGaN (indium gallium nitride), is pulsedby pulses of sufficiently high current to blue-shift the output andsufficiently short duration to not destroy the LED in order to maintaina constant light intensity while shifting the color spectrum from blueto ultraviolet. Other embodiments include IR LEDs for military or policeuse to enhance the usefulness of night-vision equipment and forfriend-or-foe identification, multiple color LEDs to produce a whitelight source, and combinations of colored LEDs to enhance the ability ofcolor-blind individuals to perceive colors. Other uses include LEDschosen for use in photographic darkrooms wherein the LED wavelength ischosen to prevent undesired exposure of light-sensitive materials.

Another embodiment of the present invention uses LEDs of various“viewing” angles to achieve wide-angle viewing versus narrow-angle,long-range viewing and combinations thereof. A further refinement ofthis embodiment utilizes a Fresnel lens (or other lens or reflectorarrangement) to provide a focusable light source. Another embodimentuses polarizers to reduce specular reflections for enhanced viewing orfor use in machine-vision applications. Another embodiment utilizesquickly and easily pluggable/replaceable LED arrays or heads of variousshapes, colors, and/or viewing angles for different applications.

In yet another embodiment, the light output is momentarily interruptedrepetitively, or strobed, to indicate low battery condition with someestimation of time to battery failure, e.g., the number of pulses couldindicate the estimated number of minutes of battery time. As theestimation of time to battery failure changes, the repetition rate isvaried to indicate impending battery failure. It is understood that thisoperational mode is easily distinguished from other operational modes bythe duration of on time versus off time. In strobe mode, low batterycondition is indicated by dropping pulses; e.g., every fourth outputpulse is dropped, or three of four pulses are dropped creating an easilydistinguishable variance in visible output of the invention.

In another embodiment of the invention, a switch is utilized to controlthe functions (and/or brightness) of the invention. A variance of thisembodiment uses a thumb-wheel, or rotary switch to vary the switchingcharacteristics to produce a variable light output.

In another embodiment, a programmable microprocessor is utilized toprovide control functionality.

FIG. 1 shows one embodiment of the present invention (a schematicrepresentation of a LED flashlight 100) having a case 110, a battery 120or other portable DC power supply, a power supply and control circuit130, a switch circuit 140, a plurality of LEDs 150, and optionally afeedback circuit 160.

In various embodiments of the present invention, feedback circuit 160(and similarly the other feedback circuits described herein) controlspulse width and/or frequency as a function of parameters such as batteryvoltage, LED light output intensity, power dissipation or devicetemperature, or LED color spectrum output.

Case 110 is any convenient size and shape, and is typically designed tohold the battery, provide a suitable grip to be handheld, and provide ahousing for the circuitry and LEDs. In one embodiment, battery 120includes one or more cells which can be any suitable technology such asalkaline dry cells or rechargeable cells. Alternatively, other portableDC electrical power sources can be used as desired in place of battery120. Power supply and control circuit (PSCC) 130 responds to a switchcircuit to apply electrical power from battery 120 to LEDs 150,controlled in order to prevent overloading and premature destruction ofLEDs 150 while minimizing power dissipation within PSCC 130, thusmaximizing battery life, providing the desired accuracy or level of theamount of light emitted at different battery voltages or otherenvironmental conditions that would otherwise vary the light output.Switch circuit 140 allows the user to control various flashlightfunctions such as, for example, on/off, setting light level, settinglight color, setting pulse or strobe frequency, and checking batteryvoltage or remaining power. In one embodiment, PSCC 130 provides a pulsetrain, in which pulse frequency, pulse width, or pulse shape/height,and/or the number of LEDs that are driven, is controlled in order toprovide a relatively constant light output level even as battery voltagedeclines and power is drained. In one embodiment, feedback circuit 160measures the light output of LEDs 150 (e.g., using a photo diode orother suitable light detecting device) and provides a signal that allowsPSCC 130 to adjust the light output to a desired level (typicallyproviding a constant light output even as battery voltage declines aspower is drained). In one such embodiment, the width of each pulse isadjusted to keep a constant average light output (widening each pulse asthe intensity of light decreases, in order to obtain a constant lightoutput). In one such embodiment, flashlight 100 is used in conjunctionwith a portable video camcorder or other video camera, and feedbackcircuit 160 measures the overall ambient light and provides a signalthat allows generation of flashlight pulses to compensate for lack oflight, in order to provide optimal lighting for the video camera. In onesuch embodiment, the pulses to the LEDs are synchronized to the videocamera frame rate using optional pulse synchronization (sync) signal 170in order that the light pulse from LEDs 150 is only on when the videocamera shutter is collecting light (avoiding light output when thecamera will not benefit from it). In another embodiment, feedbackcircuit 160 measures battery voltage, and increases pulse width,frequency, or height as battery voltage or power declines. In yetanother embodiment, feedback circuit 160 measures the current goingthrough LEDs 150, and makes the appropriate adjustment to pulse width orfrequency in order to maintain constant or desired light output.

FIG. 2 is a schematic of one embodiment of a circuit used for flashlight100. In this embodiment, normally open, momentary contact switch 146 ismomentarily closed by a user to activate light output. Power-switchcircuit 132 (in one embodiment, a TK114 circuit by Toko Americaavailable from Digikey Corporation of Thief River Falls, Minn.) isturned on as its control input 131 is shorted to ground by switch 146,thus applying voltage to Vout, which is applied through resistor divider138–138 to transistor 136. In other embodiments, circuit 132 is replacedby a simple slide switch as is used in conventional flashlights, andwhich, when closed, connects Vin to Vout (eliminating the need forresistors 137, 138, and 139, switch 146, and transistor 136). Transistor136 and resistor 137 then maintain the control voltage low enough tokeep power circuit 132 turned on even after switch 146 is released bythe user to its open position (transistor 136 is “on” as long as circuit132 is on and applying battery voltage to Vout). Thus power is appliedto Vout until an OFF signal is set high on line 131 by microprocessor(MP) 134 (resistor 137 has a resistance that is set to a value that issmall enough to keep the control pin of circuit 132 low unlessoverridden by the OFF signal from MP 134 going high). Microprocessor 134is any suitable microprocessor, such as a PIC16C62X microcontroller byMicrochip and available from Digikey Corp. of Thief River Falls, Minn.,56701. The PIC16C62X includes two analog comparators with a programmableon-chip voltage reference, a timer, and 13 input/output (I/O) pins eachcapable of direct LED driving of 25 mA source or sink. In oneembodiment, MP 134 is programmed to receive a feedback signal 260 fromfeedback circuit 160, and on the basis of the feedback signal, adjustthe drive signal(s) 250 to LEDs 150, thus adjusting the light output. Inone such embodiment, a lookup table 234 is used to convert a digitalvalue derived from feedback signal 260 into a digital value used tocontrol drive signal 250. In one embodiment, optional feature switches142 are provided to control various parameters of light output such as,for example, intensity, color, duration (i.e., time until automaticpower down), frequency (i.e., a strobe control), etc. In one embodiment,an external pulse sync signal 170 is provided, isolated though astandard opto-isolator circuit 171, and provided as an input to pulsesync input pin 270 of MP 134. In one such embodiment, pulse sync signal170 is driven from a video camera (such as a camcorder or amachine-vision video camera), in order to synchronize light output withthe light gathering/shutter open times of the camera. In another suchembodiment, pulse sync signal 170 is driven from a spark-plug-wirepickup in order to provide a timing strobe of light pulses for tuning aninternal combustion engine.

In one embodiment, feature switches 142 include momentary contactswitches in pairs, one switch of the pair used to increase a particularparameter, and the other switch of the pair used to decrease theparticular parameter (such as is done commonly in television remotecontrol devices). In one such embodiment, a pair of switchesincreases/decreases overall light output intensity. In another suchembodiment, color is adjusted, e.g., using one pair of buttons for blueLED output, another pair for green LED output, and a third pair for redLED output; or using one pair to control the X-coordinate and anotherpair to control the Y-coordinate of chromaticity (such as a CIEchromaticity diagram's X and Y coordinates). In yet another suchembodiment, a pair of switch buttons increases/decreases the remainingtimeout value. In one embodiment, as a feature switch is pressed toincrease or decrease a parameter, the number of LEDs that are “on” arevaried to provide a visual indication to the user of the value of thatparameter, for example the timeout value could be varied from one to tenminutes until power off, and as the button to increase that parameter isheld down, the timeout parameter is increased successively from one toten, and a corresponding number of LEDs (one to ten) are turned on toprovide this visual indication. In other embodiments, audibleindications of such parameters are provided, e.g., by providing variablepitch or numbers of clicks to give the user feedback as to the value ofthe parameters being adjusted or measured.

A primary feature of some embodiments of the present invention is toprovide a large number of individual LEDs in order to provide sufficientgeneralized and/or focused illumination to be useful as a handheldflashlight, or in particular, as an illumination source for a scene orobject to be imaged by a video camera (e.g., in a camcorder ormachine-vision system). With current low-cost, high intensity LEDshaving a luminous intensity of, say 2 cd, twenty to fifty LEDs aretypically needed to provide a good flashlight, although in someapplications as few as seven LEDs provide desirable results. In one suchembodiment, each individual LED is separately packaged in a transparentencapsulant (e.g., a T 1¾ package) that provides manufacturingefficiencies and provides better heat dissipation by spreading theactive light emitting chips apart from one another. In some embodiments,white LEDs are used (such as white LEDs that utilize a blue LED chip anda YAG phosphor that converts a portion of the blue light to yellow, thusyielding a white-appearing light output, such as part number NSPW 310ASavailable from Nichia Chemical Industries Ltd. of Japan and NichiaAmerica Corp., 3775 Hempland Road, Mountville Pa., 17554). In otherembodiments, standard high-efficiency colored LEDs of red, yellow,green, and/or blue are used to provide light of the desired intensityand color. In one such embodiment, LEDs of each color are controlledseparately in order to provide the desired overall hue or whiteness ofthe combined light output.

In an application such as providing illumination for a video camera,feedback circuit 160 measures the video output signal from the cameraand provides a feedback signal 260 that allows adjustment of the lightoutput pf LEDs 150 in order to optimize the video signal. In one suchembodiment, as shown in FIG. 5, the controlled LED light source isintegrated into a handheld camcorder 500. In one such embodiment, thevideo camera circuit also provides pulse sync signal 170 in order tosynchronize the light output to the video light gathering time windows.In another such embodiment, feedback circuit 160 measures the colorbalance of the video output signal, and provides separate feedbackintensity control for each of a plurality of (e.g., two or three)separate groups of color LEDs, for example, red, green, and blue. In oneembodiment, green LEDs such as part number NSPG 500S and blue LEDs suchas part number NSPB 500S, both available from Nichia Chemical IndustriesLtd. of Japan and Nichia America Corp., 3775 Hempland Road, MountvillePa., 17554 are used, and red LEDs such as part number HLMP-C115available from Hewlett Packard Company.

FIG. 3 is a circuit block diagram of an LED flashlight circuit 300,which circuit is used in some embodiments of LED flashlight 100 of FIG.1 or LED light source in video camera 500 of FIG. 5. Circuit 331replaces circuit 132 and 134 of FIG. 2 using similar circuit concepts(however, in one embodiment, the entire circuitry of circuit 331 isintegrated onto a single integrated circuit chip). When switch 146 ismomentarily closed, circuit 331 draws output pin 332 low, turning on PNPtransistor 133, which remains turned on until circuit 331 again detectsthat switch 146 is momentarily closed, at which time pin 332 is allowedto float high, turning off the flashlight circuit 300. One or moreoutput pins 336 drive one or more low-threshold high-power MOSFETs 350(e.g., a plurality of MOSFETs 350 are used to drive groups of LEDs ofdifferent colors, as described above). In some embodiments, pin 336provides a variable pulse control signal to vary pulse width, pulsefrequency, or both in order to control light output as described above.In the embodiment shown, output pin 333 is driven low to turn on LED152, and in one such embodiment, pulses LED 152 in a manner that thepulses are perceptible to the human eye, and varying the pulse patternor timing in order to indicate the estimated remaining battery power.For example, in one embodiment, from one to ten short, individuallyperceptible pulses closely spaced (e.g., one-third of a second apart)pulses are driven each time the flashlight is initially turned on, thatis, ten pulses closely spaced indicate that 100% of the battery powerremains, 9 pulses indicate that 90% of the battery power remains, . . .and 1 pulse indicates that 10% of the battery power remains. In anothersuch embodiment, LED 152 is repeatedly pulsed in this manner, e.g., tenpulses spaced at ⅓ second, then a 3 second period of time when LED 152is off, ten pulses spaced at ⅓ second, then a 3 second period of timewhen LED 152 is off, in a repeating pattern as long as the flashlight ison. In one such embodiment, this provides the user with the onlyindication of remaining battery life, since LEDs 151 are driven toprovide constant illumination, regardless of battery voltage variationsor other factors that would otherwise vary light output. In otherembodiments, feedback circuit 160 is omitted, and such factors do affectlight output. Feedback circuit 160 is as described above, and measureslight emitted by the LEDs, battery voltage, LED current, and/or otherparameters in order to provide circuit 331 with information to be usedto control output pin 336, and thus light output. In one embodiment thatmeasures light output in feedback circuit 160, the width of the pulseneeded to obtain a certain level of light output provides indirectinformation regarding remaining battery power, and is measured andconverted into the visual indication of remaining power to be displayedby LED 152. In another embodiment, the amount of remaining battery poweris visually indicated by turning on a proportional number of the LEDs151 as power is initially applied, so the user, by seeing how many LEDsare lit during this initial power-indication mode, can determine theremaining battery power. Thus, by varying the number of perceptibleflashes or the number of lit LEDs, or other visual indication, theremaining battery power can be conveyed to the user.

FIG. 4 is a circuit block diagram of an LED flashlight circuit 400,which circuit is used in some embodiments of LED flashlight 100 of FIG.1 or LED light source in video camera 500 of FIG. 5. In this circuit400, switch 147 applies (and removes) power to the circuit 400. In thisembodiment, circuit 433 provides a continuous series of very shortpulses (e.g., 10 microseconds wide each) at a frequency much higher thanthe flicker rate of the human eye (e.g., between 100 Hz to 50 KHz) thatdrive the trigger input of circuit 434 (in this embodiment, circuits 433and 434 are each 555-type timer circuits, or each are one-half of a 556dual timer). Resistor 431 and constant voltage circuit 432 provide afixed voltage to control pin 5 of circuit 555. Since circuit 434 willoperate over a wide range of voltage, as the voltage of Vout decreases,the constant voltage at pin 5 (from circuit 432) will be relativelyhigher, thus increasing the pulse width generated by circuit 434 andoutput on its pin 3. As in FIG. 3, MOSFET 350 shorts the anodes of LEDs151 to ground for the duration that pulse from circuit 434 is high. Thiscan provide 100 milliamps or more through each LED 151 when the batteryis fully charged, but only for a very short pulse. While the 100 mA, ifconstant, would overload the LEDs, the short pulses are tolerated. Asthe battery power is drained, the voltage of Vout decreases, and thepulse width increases. In this way, pulse width increases as batteryvoltage decreases, thus compensating at least partially for the reducedpeak intensity of the LEDs at lower voltage. On the other hand, LED 452is driven directly from output pin 3 of circuit 434 though currentlimiting resistor 153. LEDs 151 are “on” for proportionally longer asthe pulse width increases; however, LED 452 is on proportionally shorteras pulse width increases, thus LED 452 becomes dimmer as voltagedecreases, providing a visual indication of remaining battery power.

FIG. 5 is a diagram showing a controlled LED light source as integratedinto a handheld camcorder 500. In this embodiment, camcorder 500includes lens 520, case 510, video circuit 570, recorder apparatus 580,battery 120, control circuit 130, feedback circuit 160, and LEDs 150.Typically, lens 520 forms an image of object 599 onto a CCD imagingarray that is part of video circuit 570 (i.e., lens 520 and videocircuit 570 form a video camera), and the corresponding video signal isrecorded onto media (such as video tape or recordable digital video disk(DVD)) in recorder 580. In other applications such as machine vision,the video signal is coupled to an image processor that in turn controlssome manufacturing process, for example, and part inspection or robotarm control is accomplished. In one embodiment, feedback circuit 160takes input from the video signal only in order to control the amount oflight emitted from LEDs 150. In other embodiments, feedback circuit 160,instead of or in addition to input from the video signal, takes feedbackinput 165 from a photosensor in order to control LED light output.

FIG. 6 is a graph of color spectrum versus current for an LED to be usedin one embodiment of the present invention. As is seen in the graph, asthe LED current increases from 10 mA to 35 mA, the color spectrum ofthis exemplary LED shifts from centered at approximately 440 nanometers(blue) to centered at approximately at 380 nanometers (ultraviolet), andthe overall intensity increases with increasing current. Such an LED isdescribed by M Schauler et al, GaN based LED's with differentrecombination zones, MSR Internet Journal of Nitride Research, Volume 2Article 44, Oct. 8, 1997 (internet addresshttp://nsr.mij.mrs.org/2/44/complete.html). In one such embodiment, theabove described pulse-width control or frequency control circuits (suchas feedback circuit 160 and control circuit 134) are used to maintain adesired illumination intensity as the color spectrum is changed bychanging the current through the LED. In one such embodiment, colorbalance as measured by feedback circuit 160 is used to change thecurrent of each pulse and thus the color spectrum in order to control ormaintain color balance.

By controlling the amount of current (the height of each pulse), thecolor spectrum of the output light can be adjusted (i.e., for the abovedescribed LED, the color spectrum center wavelength is adjustable from440 nm blue to 380 nm ultraviolet), and by simultaneously controllingpulse width and/or pulse frequency, the intensity can also be controlled(i.e., one can vary the intensity, or even keep a constant intensity asthe pulse height is adjusted to change color output), e.g., by varyingpulse width to provide a constant perceived or average intensity even asthe color changes. FIG. 7 shows one circuit 700 to accomplish suchcontrol. The user controls the color desired via switches 140 coupled tocontrol circuit 730. Circuit 730 then controls the current of transistor755 by well-known techniques such as a current mirror, and the pulsewidth or frequency to transistor 750 as described above (in oneembodiment, a lookup table is used to choose a predetermined pulse widthbased on the user-selected or set color, and the current is determinedby another corresponding lookup table that is used to choose anappropriate current).

FIG. 8 shows another circuit 800 to accomplish such control. Feedbackcircuit 840 is adjusted to controls the color desired based on adetected color signal from color detector 841 in control circuit 830.Control circuit 830 then controls the current of transistor 755 by acurrent mirror with transistor 754. In addition, the pulse width orfrequency to gated register 842 is optionally controlled by one or morefeedback circuits 860 (which are controlled by a signal indicatingsupply voltage, the temperature of LEDs 151, measured light outputintensity or any other parameter over which control is desired) asdescribed above (in one embodiment, a lookup table is used to choose apredetermined pulse width based on the user-selected or set color, andthe current is determined by another corresponding lookup table that isused to choose an appropriate current). In one embodiment, gatedregister 842 receives and stores a binary number value from feedbackcircuit 840, and receives a variable-frequency and/or variable-widthoutput-enable pulse 835 from pulse-width modulator (PWM) circuit 834.PWM circuit 834 is driven by frequency generator 833. In one embodiment,both PWM circuit 834 and frequency generator 833 are set to providefixed frequency and fixed width pulses (i.e., no feedback used). Inother embodiments, one or both of frequency and pulse width arevariable, and in some embodiments, the variability is controlled byfeedback, and in other embodiments these parameters are set-able tovalues chosen by a user. In some embodiments, a maximum frequency for agiven pulse width, or a maximum pulse width for a given frequency ispredetermined in order to prevent destruction of LEDs 151 from excessivepower. In other embodiments, a temperature feedback signal indicatingthe temperature of LEDs 151 is coupled to feedback circuit 860 toprevent overheating of LEDs 151. In some embodiments, feedback circuit860 simply inhibits and/or shortens pulses based on temperature feedbackor on a predetermined maximum rate limit or pulse-width limit. FETtransistors 851, 852, 853, and 854, and their respective resistors R,2R, 4R, and 8R, along with trimming resistor 859 form a controllablevariable current source, which is multiplied by approximately the factorβ/(β+2) by the current mirror of transistors 754 and 755 to get the sumof the current though LEDs 151. For optimal results, transistors 754 and755 are formed as a single integrated three-terminal device on a singlesubstrate, in order to achieve matched betas and temperature dependence.In one such embodiment, transistor 755 is formed of multiple individualtransistors wired in parallel in order to achieve higher output current(at the collector of transistor 755) for a given input current (at thecollector of transistor 754 and the bases of the two transistors). Inthe embodiment shown, this current mirror allows circuit 830 todetermine the current through transistor 755 substantially independentof the voltage of Vcc and the voltage-current relationships of LEDs 151.In the embodiment shown, LEDs 151 are wired in parallel; however, inother embodiments a single LED device is used, or a plurality of LEDs151 are instead wired in series, or in a series-parallel arrangement asis shown in FIG. 10.

FIG. 9 is a graph of color spectrum (photoluminescence) versustemperature for an LED to be used in one embodiment of the presentinvention. Such an LED is described by B. Monemar et al, Free Excitonsin GaN, MSR Internet Journal of Nitride Research, Volume 1 Article 2,Jul. 8, 1996 (internet addresshttp://nsr.mij.mrs.org/1/2/complete.html). In one embodiment of thepresent invention, a GaN or InGaN LED that exhibits atemperature-dependent color spectrum light output (i.e.,electroluminescence, or light output due to a current flowing throughthe LED which also exhibits color-temperature dependence, as opposed tothe photoluminescence graphed in FIG. 9) has its color spectrumcontrolled by one of the circuits described for FIGS. 1, 2, 3, 4, 5, 7,or 8. In one such embodiment, feedback reduces or eliminates colorchanges that would otherwise occur as temperature of the LED changed. Inanother such embodiment, color changes are purposely induced by changingthe temperature of the LED, either by heating or cooling the LED with anexternal temperature-change device such as a resistor, or by inducinginternal temperature changes by changing the average driving current toeffect a change in junction temperature in the LED. In some suchembodiments, a color detector such as is well known in the art is usedto provide a signal to provide feedback to control temperature.

FIG. 10 is a block diagram of the control circuit 130 for one embodimentof illumination system 100. An Oscillator 15 (in the embodiment shown,oscillator 15 is part of an image processor as shown which is coupled toan electronic camera 14, e.g., a charge-coupled device (CCD)),controlled by pulse-frequency circuit (such as circuit 833 of FIG. 8 orfrequency generator 433 of FIG. 4), sends a trigger signal to powersupply 20. In one embodiment, the image processor 15 generates one pulseor a plurality of pulses for each CCD frame, wherein the number ofpulses generated is sufficient to provide a desired accumulation oflight received by camera 14 for each frame. Within power supply 20, thetrigger signal activates pulse generator 201 to generate a control pulseof a length determined by pulse-length circuit (such as PWM circuit 834of FIG. 8 or PWM 434 of FIG. 4) The control pulse is used to turn ontransistor Q1 to generate a flash on LEDs 25, which is current-limitedby resistor R3. The control pulse also activates the maximum-rate-limitcircuit 202, which inhibits any further control pulses from pulsegenerator 201 for a predetermined amount of time. The 12 volt signalfrom power supply 20 is filtered by the low-pass filter comprising C1,L1, D1, and R1, and charges capacitors C2 through C_(N) (in oneembodiment, N is 12). In one such embodiment, C1 through C12 are each2200 μF, L1 is 40 μH iron-core, D1 is a 1N4001 diode, and R1 is a 0 ohmconductor. C2 through C_(N) are discharged through fifteen series-wiredLEDs 25, which in this embodiment are wired in a parallel-series manneras shown, and R3 and Q1, as activated by the above-described controlpulse. In one such embodiment, R3 is replaced by a zero-ohm conductor,and the voltage drop across the LEDs and Q1 is used to self-limit thecurrent through the LEDs. The control pulse is fed across resistor R2,which in one embodiment is 100KΩ, to develop the necessary voltage fordriving transistor Q1, which in this embodiment is a MTP75N05HD MOSFET.

FIG. 11 is a more-detailed schematic diagram of power supply 20. Theinput trigger is fed through resistor R2 to drive the input ofopto-isolator OI1. The output of opto-isolator OI1 is coupled throughcapacitor C12 (and the associated circuit R4, R6 and D2) to the TRGinput of timer circuit 1C1 _(A). (In one embodiment, timers 1C1 _(A) and1C1 _(B) are each ½ of a 556-type dual timer.) The timing constant oftimer 1C1 _(A) is set by C14 and R1-x, (where x is selected from 1through N), and determines the pulse width of the control pulse drivingQ1, and thus the LEDs. In one embodiment, five selectable pulse widthsare predetermined and selected by SW1, which is a five-way exclusivedual-pole-single-throw switch, wherein one resistor of the set R1-1through R1-N is selected for connection to the DIS input pin of 1C1_(A), and a corresponding one resistor of the set R2-1 through R2-N isselected for connection to the DIS input pin of 1C1 _(B). The timingconstant of timer 1C1 _(B) is set by C17 and R2-x, (where x is selectedfrom 1 through N), and determines the minimum time between controlpulses driving Q1, and thus the LEDs. In one embodiment, the fiveselectable predetermined pulse widths are 25 microseconds (μs), 50 μs,100 μs, 200 μs and 500 μs; the corresponding maximum pulse ratescontrolled by maximum rate limit circuit 202 are 200 Hz, 120 Hz, 60 Hz,30 Hz, and 10 Hz, respectively, and are predetermined and selected bySW1. Thus, in the embodiment which uses a 60 Hz camera image rate, 100μs-long control pulses are used to activate LEDs 25. In one embodiment,it is desired to have an average LED illumination intensity of at leastten times the ambient light; thus, when imaging device 14 is taking oneframe every 16.7 milliseconds, a 100 microsecond pulse should be atleast 1670 times as intense as the ambient light. In one such anembodiment, a shroud is used to reduce the ambient light, and a redfilter (substantially transparent to the peak wavelength of illuminationsource 18) is placed over the lens of imaging device 14 in order toreduce ambient light and pass the light of illumination source 18. Thecontrol pulse output signal is driven through resistor R3.

In one embodiment, opto-isolator OI1 is a 4N37-type part, resistor R2 is100 Ω, resistor R3 is 100 Ω, resistor R7 is 1MΩ, resistor R8 is 1KΩ andvisible-color LED D3 indicates when the circuit is active, resistor R4is 4700Ω, resistor R5 is 10Ω, resistor R6 is 10KΩ, diode D2 is a 1N914,resistor R1-1 is 2.26KΩ, resistor R1-2 is 4.53KΩ, resistor R1-3 is9.1KΩ, resistor R1-4 is 18.2KΩ, resistor R1-5 is 45.3KΩ, resistor R2-1is 37.4KΩ, resistor R2-2 75Ω, resistor R2-3 is 150KΩ, resistor R2-4 is301KΩ, resistor R2-5 is 909KΩ, C14 is 0.01 μF, C17 is 0.1 μF, C12 is0.001 μF, C10 is 100 μF, C11 is 0.1 μF, C13, C15, and C16 are each 0.01μF, Q2 and Q3 are each 2N3904 NPN transistors, and RP1 is a 10KΩresistor pack.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A method of providing changeable illumination of such compact sizeand low weight as to be suitable for single-handed portable operation bya user, comprising: providing one or more LEDs in a housing, the LEDshaving a first characteristic color spectrum output and one or more LEDshaving a second characteristic color spectrum output, wherein the firstcharacteristic color spectrum output is different from the secondcharacteristic color spectrum output; selectively applying pulsed powerfrom a DC-battery-powered voltage source to the LEDs, wherein the pulsesare of high-enough frequency such that the human eye does not perceivethe pulses; projecting light to illuminate an object separate from thehousing; and changing a pulse characteristic of the pulsed power inorder to change a proportion of light output having the firstcharacteristic color spectrum output to that having the secondcharacteristic color spectrum output.
 2. The method of claim 1, whereinthe changing of the pulse characteristic increases light output level ofthe LEDs having a first characteristic color spectrum output byincreasing a pulse width.
 3. The method of claim 1, wherein the changingof the pulse characteristic increases light output level of the LEDshaving a first characteristic color spectrum output by increasing apulse energy.
 4. The method of claim 1, wherein the changing of thepulse characteristic increases light output level of the LEDs having afirst characteristic color spectrum output by increasing a pulsefrequency.
 5. The method of claim 1, wherein the changing of the pulsecharacteristic maintains an average predetermined light output level ofthe LED units by sensing a battery voltage and adjusting a pulse widthto the LED units based on the measured voltage to maintain an averagelight output at a predetermined level.
 6. The method of claim 1, furthercomprising: providing one or more LEDs having a third characteristiccolor spectrum output, wherein the third characteristic color spectrumoutput is blue LED output, the second characteristic color spectrumoutput is green LED output, and the first characteristic color spectrumoutput is red LED output.
 7. The method of claim 6, further comprising:using one pair of buttons for blue LED output, a second pair of buttonsfor green LED output, and a third pair of buttons for red LED output. 8.The method of claim 6, further comprising: using one pair of buttons tocontrol the X-coordinate and another pair of buttons to control theY-coordinate of chromaticity.
 9. The method of claim 6, furthercomprising: using information from a first user input device to controlblue LED output, a second user input device to control green LED output,and a third user input device to control red LED output.
 10. The methodof claim 6, further comprising: using information from a first userinput device to control the X-coordinate of chromaticity and a seconduser input device to control the Y-coordinate of chromaticity.
 11. Amethod of providing changeable illumination of such compact size and lowweight as to be suitable for single-handed portable operation by a user,comprising: providing one or more LEDs having a first characteristiccolor spectrum output and one or more LEDs having a secondcharacteristic color spectrum output, wherein the first characteristiccolor spectrum output is different from the second characteristic colorspectrum output; selectively applying pulsed power from a DC voltagesource to the LEDs, wherein the pulses are of high-enough frequency suchthat the human eye does not perceive the pulses; and changing a pulsecharacteristic of the pulsed power in order to change a proportion oflight output having the first characteristic color spectrum output tothat having the second characteristic color spectrum output, wherein thechanging of the pulse characteristic maintains an average predeterminedlight output level of the LED units by sensing a battery voltage andadjusting a pulse characteristic to the LED units based on the measuredvoltage to maintain an average light output at a predetermined level.12. A method of providing changeable illumination of such compact sizeand low weight as to be suitable for single-handed portable operation bya user, comprising: providing one or more LEDs having a firstcharacteristic color spectrum output and one or more LEDs having asecond characteristic color spectrum output, wherein the firstcharacteristic color spectrum output is different from the secondcharacteristic color spectrum output; selectively applying pulsed powerfrom a DC voltage source to the LEDs, wherein the pulses are ofhigh-enough frequency such that the human eye does not perceive thepulses; and changing a pulse characteristic of the pulsed power in orderto change a proportion of light output having the first characteristiccolor spectrum output to that having the second characteristic colorspectrum output, wherein the changing of the pulse characteristicmaintains an average predetermined light output level of the LED unitsby sensing a light output and adjusting a pulse characteristic to theLED units based on the measured voltage to maintain the average lightoutput at the predetermined level.
 13. A portable pulsed LEDillumination source of such compact size and low weight as to besuitable for single-handed portable operation by a user, comprising: oneor more LEDs having a first characteristic color spectrum output; one ormore LEDs having a second characteristic color spectrum output, whereinthe first characteristic color spectrum output different from the secondcharacteristic color spectrum output; a housing that holds the one ormore LEDs having the first characteristic color spectrum output and theone or more LEDs having the second characteristic color spectrum output,wherein the LED illumination source is configured to project light toilluminate an object separate from the housing; a battery connectorconfigured to electrically connect to one or more batteries; and acontrol circuit that is operatively coupled to receive electrical powerfrom the battery connector and to supply electrical power to the LEDs,and that controls a pulse characteristic to the one or more LEDs havinga first characteristic color spectrum output in order to change aproportion of light output having the first characteristic colorspectrum output to that having the second characteristic color spectrumoutput.
 14. The illumination source of claim 13, wherein the controlcircuit increases light output level of the LEDs having a firstcharacteristic color spectrum output by increasing a pulse width. 15.The illumination source of claim 13, wherein the control circuitincreases light output level of the LEDs having a first characteristiccolor spectrum output by increasing a pulse energy.
 16. The illuminationsource of claim 13, wherein the control circuit increases light outputlevel of the LEDs having a first characteristic color spectrum output byincreasing a pulse frequency.
 17. The illumination source of claim 13,wherein the control circuit increases light output level of the LEDshaving a first characteristic color spectrum output by sensing a voltageand adjusting a pulse width based on the sensed voltage.
 18. Theillumination source of claim 13, wherein the control circuit increaseslight output level of the LEDs having a first characteristic colorspectrum output by sensing a voltage and adjusting a pulse based on thesensed voltage.
 19. The illumination source of claim 13, wherein thecontrol circuit increases light output level of the LEDs having a firstcharacteristic color spectrum output by sensing a voltage and adjustinga pulse frequency based on the sensed voltage.
 20. The illuminationsource of claim 11, further comprising a signal-controlling circuitoperatively coupled to one or more of the LEDs to generate a visiblyinterrupted pulse train that includes a coded sequence “SOS” in Morsecode.
 21. The illumination source of claim 13, wherein the controlcircuit is operatively coupled to the LEDs and selectively applies powerpulses from a DC voltage source to the LEDs, the control circuitsubstantially maintaining an average light output characteristic of theLEDs as a voltage of the voltage source varies over a range that wouldotherwise vary the light output characteristic by adjusting the pulses,and wherein the light output characteristic that is maintained is lightoutput intensity.
 22. The illumination source of claim 21, wherein thecontrol circuit increases light output level of the LEDs having a firstcharacteristic color spectrum output by increasing a pulse width. 23.The illumination source of claim 21, wherein the control circuitincreases light output level of the LEDs having a first characteristiccolor spectrum output by increasing a pulse energy.
 24. The illuminationsource of claim 21, wherein the control circuit increases light outputlevel of the LEDs having a first characteristic color spectrum output byincreasing a pulse frequency.
 25. The illumination source of claim 21,wherein the control circuit increases light output level of the LEDshaving a first characteristic color spectrum output by sensing a voltageand adjusting a pulse width.
 26. The illumination source of claim 21,wherein the control circuit increases light output level of the LEDshaving a first characteristic color spectrum output by sensing a voltageand adjusting a pulse.
 27. The illumination source of claim 21, whereinthe control circuit increases light output level of the LEDs having afirst characteristic color spectrum output by sensing a voltage andadjusting a pulse frequency.
 28. A battery-powered portable pulsed LEDillumination source of such compact size and low weight as to besuitable for single-handed portable operation by a user, the sourcecomprising: one or more LEDs having a first characteristic colorspectrum output; one or more LEDs having a second characteristic colorspectrum output, wherein the first characteristic color spectrum outputdifferent from the second characteristic color spectrum output; ahousing that holds the one or more LEDs having the first characteristiccolor spectrum output and the one or more a LEDs having the secondcharacteristic color spectrum output, wherein the housing and the LEDsare configured to project light to illuminate an object; a switch; andcontrol means controlled by the switch and operatively coupled to theLEDs for changing a proportion of light output having the firstcharacteristic color spectrum output to that having the secondcharacteristic color spectrum output, wherein the control means drivesthe LEDs with electrical pulses at a frequency high enough that lightproduced by the LEDs has an appearance to a human user of beingcontinuous rather than pulsed.
 29. The illumination source of claim 28,wherein the control means further comprise: feedback means forcontrolling the pulses so that light intensity produced by the LEDs, asperceived by the human user, is substantially constant across a greaterrange of battery power than a corresponding range for which lightintensity is equally constant without the feedback circuit.
 30. Theillumination source of claim 28, wherein the control means increaseslight output level of the LEDs having a first characteristic colorspectrum output by increasing a pulse width.
 31. The illumination sourceof claim 28, wherein the control means increases light output level ofthe LEDs having a first characteristic color spectrum output byincreasing a pulse energy.
 32. The illumination source of claim 28,wherein the control means increases light output level of the LEDshaving a first characteristic color spectrum output by increasing apulse frequency.
 33. The illumination source of claim 28, wherein thecontrol means increases light output level of the LEDs having a firstcharacteristic color spectrum output by sensing a voltage and adjustinga pulse width based on the sensed voltage.
 34. The illumination sourceof claim 28, wherein the control means increases light output level ofthe LEDs having a first characteristic color spectrum output by sensinga voltage and adjusting a pulse based on the sensed voltage.
 35. Theillumination source of claim 28, wherein the control means increaseslight output level of the LEDs having a first characteristic colorspectrum output by sensing a voltage and adjusting a pulse frequencybased on the sensed voltage.
 36. A portable pulsed LED illuminationsource of such compact size and low weight as to be suitable forsingle-handed portable operation by a user, comprising: one or more LEDshaving a first characteristic color spectrum output; a user-input switchcircuit that receives a light-output-level-setting input indication froma user; and a control circuit that controls a pulse characteristic ofthe electrical power to the one or more LEDs having a firstcharacteristic color spectrum output in order to provide user-selectablecontrol of the pulse frequency and/or the pulse width to obtain areduced apparent brightness due to a change of an “on” proportion oflight output having the first characteristic color spectrum output basedon the light-output-level-setting input.
 37. The illumination source ofclaim 36, further comprising: one or more LEDs having a secondcharacteristic color spectrum output, wherein the first characteristiccolor spectrum output is different from the second characteristic colorspectrum output, wherein the user-input switch circuit receives alight-output-color-setting input indication from a user and wherein thecontrol circuit changes an overall color spectrum by changing theproportion of light output having the first characteristic colorspectrum output to that having the second characteristic color spectrumoutput based on the light-output-color-setting input.
 38. Theillumination source of claim 36, further comprising a signal-controllingcircuit operatively coupled to one or more of the LEDs to generate avisibly interrupted pulse train that includes a coded sequence “SOS” inMorse code.
 39. The illumination source of claim 36, wherein the controlcircuit is operatively coupled to the LEDs and selectively applies powerpulses from a DC voltage source to the LEDs, the control circuitsubstantially maintaining an average light output characteristic of theLEDs as a voltage of the voltage source varies over a range that wouldotherwise vary the light output characteristic by adjusting the pulses,and wherein the light output characteristic that is maintained is lightoutput intensity.